A charge pump circuit uses an input voltage to generate an output voltage that is higher in level than the input voltage. A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pumps use some form of switching device(s) to control the connection of voltages to the capacitor. For instance, to generate a higher voltage, the first stage involves the capacitor being connected across a voltage and charged up. In the second stage, the capacitor is disconnected from the original charging voltage and reconnected with its negative terminal to the original positive charging voltage. Because the capacitor retains the voltage across it (ignoring leakage effects) the positive terminal voltage is added to the original, effectively doubling the voltage.
The pulsing nature of the higher voltage output is typically smoothed by the use of an output capacitor. This is the charge pumping action, which typically operates at tens of kilohertz up to several megahertz to minimize the amount of capacitance required. The capacitor used as the charge pump is typically known as the “flying capacitor.” Another way to explain the operation of a charge pump is to consider it as the combination of a DC to AC converter (the switches) followed by a voltage multiplier. The voltage is load-dependent; higher loads result in lower average voltages. Charge pumps can double voltages, triple voltages, halve voltages, invert voltages, fractionally multiply or scale voltages such as ×3/2, ×4/3, ×2/3, etc. and generate arbitrary voltages, depending on the controller and circuit topology.
Capacitive voltage conversion is achieved by switching a capacitor periodically. Passive diodes can perform this switching function in the simplest cases, if an alternating voltage is available. Otherwise, DC voltage levels require the use of active switches, which first charge the capacitor by connecting it across a voltage source and then connect it to the output in a way that produces a different voltage level.
FIG. 1 provides a common integrated circuit 100 using this principle, which may be considered to be the prototype of the classic charge pump. Circuit 100 integrates switches and an oscillator so that switches 110, 130 and 120, 140 work alternately. The configuration shown in FIG. 1 inverts input voltage 105. With a slight change in the external connections, circuit 100 can double or divide the input voltage as well. Closing switch 110 and switch 130 charges flying capacitor 150, to input voltage 105 in the first half cycle of the oscillator period. In the second half cycle of the oscillator period, switch 110 and switch 130 open and switch 120 and switch 140 close. This action connects the positive terminal of capacitor 150 to ground and connects the negative terminal of capacitor 150 to output to VOUT 180. Capacitor 150 is then in parallel with the reservoir capacitor 160.
If the voltage across capacitor 160 is smaller than that across capacitor 150, charge flows from capacitor 150 to capacitor 160 until the voltage across capacitor 160 reaches the inverse of input voltage 105. An integrated fixed-frequency oscillator drives the periodic switching in circuit 100. This circuit has no output regulation, and the switching frequency remains constant for all loads. Thus, the output voltage variation depends strongly on the load. With no load, the output voltage corresponds to the negative input voltage: VOUT=−(V+). As the output load increases, the magnitude of VOUT increases. Since VOUT is actually the inverse of VIN, a load to 0V would tend to increase vout (VOUT then being less negative).
Output current for this type of circuit may, therefore, be limited. This is partly due to a low oscillator frequency, and partly due to integrated analog switches which are far from ideal. These switches in the “on” state exhibit several ohms of on-resistance. Additionally, charge pumps are generally power inefficient. There are heretofore unaddressed needs with previous charge pump circuits.